Late-Night Loudness: How to Calm Your Noisy Ducts and Furnace

Dec 18, 2014

Without the furnace and the ductwork connected to it, you wouldn’t be able to keep your Highland Park home warm this winter. Still, mysterious furnace and duct noises are a nuisance. Learn the most likely reasons your home’s furnace and ducts make these sounds and how to calm them down.

‘Banging’ or ‘Popping’ Noises

When the thermostat calls for heat, the furnace burners turn on and the blower motor begins to circulate air through the ductwork. Pressure and heat within the ducts increase, causing the metal to expand. This expansion creates the banging or popping duct noise you hear. While it can be annoying, there’s nothing dangerous about these sounds coming from the ductwork.

Ducts are most likely to make this noise if they’re rectangular shaped. Unlike square or round ductwork, rectangular ducts have two long, weaker sides that “pop” at a lower heat and pressure. The ductwork thickness is a factor, as well. Lower gauged sheet metal is thicker and therefore less likely to pop under pressure.

The most thorough way to do away with this duct noise is to replace high-gauge rectangular steel ducts with square or round versions made from lower-gauge steel. However, this may not be practical or affordable. Another option is to add a crease or bend on the long sides of rectangular ducts. Called cross-breaking the ductwork, adding this crease stiffens and strengthens the sheet metal so it can withstand a higher pressure without popping.

‘Whistling’ Noises

You may hear whistling from individual air vents in your home as a result of air pressure behind the grill. Try adjusting the louvers so the air can more easily pass through the register. If this doesn’t work, replace the register with one properly sized for the cubic feet per minute (CFM) of the airflow passing through the vent. If these efforts don’t work, you can have a heating and cooling technician adjust the blower motor fan speed to lower the CFM.

Loose duct connections may also make whistling sounds. Listen closely to where the noise is coming from and shine a flashlight into accessible areas to check the quality of each ductwork section. If the noise is coming from somewhere out of reach, leave the inspection to a professional. The solution is to find the loose joint and tighten or seal it with duct mastic.

The furnace itself may also generate a whistling noise. This indicates air is escaping from somewhere in the system. A new or rebuilt fan set to a higher CFM than usual could be the source of the furnace noise. Decreasing the fan speed is the solution in this instance as well.

‘Booming’ Sounds When the Burners Ignite

The first possible cause is that you haven’t had the furnace cleaned in a few years and one or more burners have collected dust, dirt and other debris. This results in delayed ignition.

When the thermostat calls for heat, natural gas is supposed to ignite the burners. In a properly functioning system, all five or six burners ignite simultaneously. If one or more burners are blocked by dirt, debris or rust, the ignition could take several seconds or even minutes to occur, finally igniting only when enough natural gas enters the furnace. This delayed ignition results in a small explosion, creating the booming furnace noise you hear.

If you ignore this explosive sound and continue to run the furnace, you increase the chance of causing a crack in the heat exchanger. If this happens, you will likely need to replace the entire unit. Delayed ignition is also a hazard that could cause a house fire.

The solution is to hire a heating and cooling professional to clean the burners and other furnace components. Plan to schedule preventative maintenance for the furnace once every fall to prevent this problem from recurring.